Question

What is anodising? Give its application?

Answer 1

Anodising is used to form a thick layer of oxide which range from 5 to 30 $\mu m$on aluminium metal which provide the aluminium metal a good resistivity towards corrosion, high durability and high strength. Anodising is referred to as the electrolytic process where the metal acts as the anode.

Complete step by step answer:
The anodising is defined as the electrolytic process which is used to produce thick oxide coating usually done in aluminium and its alloy. The oxide layer formed is 5 to 30 $\mu m$ in thickness. This helps to give improved surface resistance to wear and corrosion resistance.
In the electrolytic process, the metal which is treated is considered as anode in a dilute acidic solution. The oxidation reaction takes place on the metal surface and as a result form an oxide layer. Before the electrolytic process, the aluminium is given a pre-treatment. This pre-treatment will help to enhance the properties of anodising coating. The pre-treatment includes, polishing, machining, drilling, welding.
Some application of anodising is shown below.
(1) Anodising provides a protective coating on the metal surface.
(2) Anodising steel forms a black film which is used as decorative articles.
(3) Anodising zinc gives several colour coatings.
(4) Anodising on silver jewellery.

Note:
Mainly three types of electrolytic solution is used in anodising process. The first is a 10-15% solution of sulphuric acid at 25 degree Celsius. The second electrolytic solution is sulphuric acid and oxalic acid mixture at 30 degree Celsius. The third electrolytic solution is 10% chromic acid at 38-42 degree Celsius.